package org.apache.lucene.store;

/**
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.index.IndexWriter;

import java.io.IOException;

/** An interprocess mutex lock.
 * <p>Typical use might look like:<pre>
 * new Lock.With(directory.makeLock("my.lock")) {
 *     public Object doBody() {
 *       <i>... code to execute while locked ...</i>
 *     }
 *   }.run();
 * </pre>
 *
 * @author Doug Cutting
 * @version $Id: Lock.java 179414 2005-06-01 20:10:58Z dnaber $
 * @see Directory#makeLock(String)
 */
public abstract class Lock {
    public static long LOCK_POLL_INTERVAL = 1000;

    /** Attempts to obtain exclusive access and immediately return
     *  upon success or failure.
     * @return true iff exclusive access is obtained
     */
    public abstract boolean obtain() throws IOException;

    /** Attempts to obtain an exclusive lock within amount
     *  of time given. Currently polls once per second until
     *  lockWaitTimeout is passed.
     * @param lockWaitTimeout length of time to wait in ms
     * @return true if lock was obtained
     * @throws IOException if lock wait times out or obtain() throws an IOException
     */
    public boolean obtain(long lockWaitTimeout) throws IOException {
        boolean locked = obtain();
        int maxSleepCount = (int) (lockWaitTimeout / LOCK_POLL_INTERVAL);
        int sleepCount = 0;
        while (!locked) {
            if (sleepCount++ == maxSleepCount) {
                throw new IOException("Lock obtain timed out: " + this.toString());
            }
            try {
                Thread.sleep(LOCK_POLL_INTERVAL);
            } catch (InterruptedException e) {
                throw new IOException(e.toString());
            }
            locked = obtain();
        }
        return locked;
    }

    /** Releases exclusive access. */
    public abstract void release();

    /** Returns true if the resource is currently locked.  Note that one must
     * still call {@link #obtain()} before using the resource. */
    public abstract boolean isLocked();

    /** Utility class for executing code with exclusive access. */
    public abstract static class With {
        private Lock lock;
        private long lockWaitTimeout;

        /** Constructs an executor that will grab the named lock.
         *  Defaults lockWaitTimeout to Lock.COMMIT_LOCK_TIMEOUT.
         *  @deprecated Kept only to avoid breaking existing code.
         */
        public With(Lock lock) {
            this(lock, IndexWriter.COMMIT_LOCK_TIMEOUT);
        }

        /** Constructs an executor that will grab the named lock. */
        public With(Lock lock, long lockWaitTimeout) {
            this.lock = lock;
            this.lockWaitTimeout = lockWaitTimeout;
        }

        /** Code to execute with exclusive access. */
        protected abstract Object doBody() throws IOException;

        /** Calls {@link #doBody} while <i>lock</i> is obtained.  Blocks if lock
         * cannot be obtained immediately.  Retries to obtain lock once per second
         * until it is obtained, or until it has tried ten times. Lock is released when
         * {@link #doBody} exits. */
        public Object run() throws IOException {
            boolean locked = false;
            try {
                locked = lock.obtain(lockWaitTimeout);
                return doBody();
            } finally {
                if (locked)
                    lock.release();
            }
        }
    }

}
